Yeah, good luck with that. Our bar for releasing Alpha was 'it can successfully install by nuking everything else on the drive'. Anything beyond that is bonus points. =) You will need to use custom partitioning to do anything other than blowing away the selected disk, and custom partitioning is very break-y. You might be able to get it to work, but I wouldn't expect too much.
I suppose it's worth saying this again: ALPHA RELEASE IS ALPHA
Alpha does not mean 'finished'. Alpha does not even mean 'nearly finished'. Alpha means 'here's this thing that will probably eat your babies and burn down your house, we want you to tell us exactly where the teeth marks were on the babies and what the fire smelled like'. Srsly. Just because we sometimes manage to crank out Alphas that work rather well doesn't mean it's the usual state of affairs (and is usually simply a consequence of not much changing in the really critical parts of the distro). When there are major changes to anaconda, expect Alpha to be rickety. It's the way things are meant to be.
You cannot yet judge Fedora 18 as Fedora 18 does not exist. You can judge Fedora 18 Alpha. It's very much not the same thing.
So in case it's not known yet: 18 Alpha RC3 will go out as Alpha (not RC3.1, that was a test spin which we didn't take in the end). There are still known major issues in it. Please read the common bugs list and release announcement before installing it, and install it only on test systems you do not care about. As the common bugs list and release announcement don't exist yet, following these instructions will involve waiting. You choose not to follow 'em at your own peril.